The safety valve comes into play when all the other protections have failed to do their job. Normally the high-pressure switch stops the compressor. But if a fire, a vessel completely full of liquid and closed at both ends, or a faulty pressure switch keeps raising the pressure, the disc held by the spring opens and discharges refrigerant. When the pressure falls the disc closes again. It is found on the receiver, the oil separator, large suction accumulators and every section of a CO₂ system that can be isolated.
The set pressure of the valve cannot exceed the maximum allowable pressure (PS) written on the nameplate of the vessel it protects. For HFC condensing units 25–33 bar on the high-pressure side, 40–45 bar for R410A and 120–140 bar in transcritical CO₂ systems are typical values. The high-pressure switch is set below this, usually so as not to exceed 90% of the set pressure, so that the valve opens only in a real emergency. Its nameplate states the set pressure, the discharge capacity and the date of manufacture; a valve without a nameplate is not used.
The discharge capacity of the valve is calculated according to the size of the vessel; for refrigeration systems this calculation is defined in the EN 13136 standard and the general rules in EN 378. The outlet is directed to the open air where no people are present; the outlet pipe is not reduced in size and is made so that rainwater cannot fill it. On large vessels twin safety valves on a three-way changeover valve are used; while one is being removed the other continues to protect the vessel. Refrigerant discharged into an enclosed machinery room creates a danger of asphyxiation; with flammable and toxic refrigerants this rule is even more important.
A safety valve that has opened once mostly does not close fully and leaks slightly; as a rule it is replaced. The setting of the valve is not tampered with, its seal is not removed and no shut-off valve is placed between the valve and the vessel. A trace of oil at the outlet is the sign of a leak. Valves are inspected or renewed at the intervals laid down by the manufacturer and by the pressure equipment legislation in force; in practice this period is mostly around five years. A replacement valve must have the same set pressure as the old one and at least the same capacity.